A Boba Fett movie could work. He's a potentially interesting character with enormous room for fresh stories between what little has been established on screen. (And bear in mind, for most of the movie going public, the movies are all that count, not any extended universe, novels, comic books, etc.).

But a Yoda movie or a Solo movie is only going to give us one of two things. It will either give us revelations about their past that diminish, rather than enhance, the mystery; or it will give us silly contradictions to established canon. Possibly both.

If it were me, I'd do something really off-axis and run with Yoda's gnomic comment about "there is another". Yes, I know it was originally supposed to refer to Leia, but that idea never really got played out (again, only counting the movies). I'd love to see a writer take the idea that in the years that she and Anakin were apart, Amidala had a child out of wedlock that she gave up for adoption -- and that's the sister that Yoda was referring to. With no training nor moral guidance, nor indeed any knowledge of the Jedi, she grows up to have strange powers but no idea how to properly control them, so she becomes -- what? A con artist? A high-end burglar, the Catwoman of the Star Wars universe? Perhaps she is hunted by Imperial agents who recognize her Force powers, but she has absolutely no idea why.

A Boba Fett movie won't work for the same reason that the Wolverine movie sucked. The appeal of the character is in how little you know of him, that he comes in and out of the main action, influencing events for his own agenda which is distinct and alien to the agenda of both the protagonists AND the antagonists. Sometimes a character is compelling because they are a sidetrack, a distraction, a wild card.